The invention relates to a method for actuating electromechanical transducers for the purpose of generating a prescribed power characteristic or torque characteristic, in particular for reducing angle-dependent torque fluctuations in electric motors, in which time-dependent or position-dependent (travel-dependent or angle of rotation-dependent) data sets are stored in a function memory, which data sets are called up as a function of the travel or angle of rotation covered in operation or with timing control and are logically connected in an arithmetic switching unit to an input variable to form momentary values, and in which, as a function of the momentary values, voltages or currents with corresponding time-dependent or position-dependent curve shape are impressed into the electrical terminals of the transducer. In addition, an arrangement for carrying out such a process is a subject of the present invention.
An important property, for example of an electric motor, is its concentricity quality (or the uniform power characteristic of an electromechanical transducer). It influences both the accuracy and the stability of a drive system. In order to be able to suppress the disturbing torque pulsations in motors, it is first necessary to localize the cause. Four factors are essentially responsible for the torque fluctuations:
Already with a currentless armature, permanent-magnet torque fluctuations arise, triggered by the interaction of the permanently magnetic materials and the winding grooves or other ferromagnetic components, in motors with permanent-magnet excitation or in motors with iron parts having high residual induction. A rotation of the rotor leads to fluctuations of the overall energy of the magnetic circuit and thus to angle-dependent torques with alternating stable and unstable extreme values.
In contrast with this, the electromagnetic torque fluctuations arise from the interaction of the armature electric loading and the magnetic field. The electromagnetic fluctuations are a result of the special distribution of magnetic fields in the air gap, the winding arrangement and the armature electric loading curve shape as a function of the angle of the rotor.
An angle-dependent change in the motor inductance, as occurs for example with a non-uniform air gap, with partial iron saturation, with a non-uniform material distribution, with respect to the magnetic permeance, and other effects, leads in conjunction with the armature currents to reluctance torque fluctuations.
Torque pulsations in the motor can also have mechanical causes. The mechanical torque fluctuations, as they will be referred to below for the sake of simplicity, are triggered for example by unsymmetrical stresses of the motor shaft such as axle shifts at couplings, eccentric bearing seats etc. They can also result from the load coupled to the motor (or generally transducer).
As a rule, all four types of torque fluctuation referred to occur together in the electric motor but usually with a different order of magnitude of the individual components. There are cases in which individual components are negligible with respect to the others.
Efforts have already been made to improve the concentricity quality of electric motors by constructional measures.
The portion of the permanent-magnet torque fluctuations can be eliminated for example by using a non-iron-containing winding with an annular magnetic yoke (for example: bell-type armature motors). A considerable reduction is already achieved by placing the iron laminated core at an angle, for example by one slot pitch, and by a suitable design of the shape of the magnet and of the slot, tooth or poleshoe geometry. Drive motors which are designed for steady-state motor speeds are frequently equipped with an additional flyweight (for example, record players).
The electromagnetic pole sensitivity (pole cogging) can be favorably influenced for example by means of a selection of the winding design matched to the air gap field and the current curve and thus also by inclining the slot pitch.
The reluctance torque fluctuations can be considerably reduced, inter alia, by using rotationally symmetrically arranged low-retentivity and high-retentivity materials.
However, these known constructional measures for improving the concentricity quality or corresponding measures for achieving a uniform power characteristic of a general, electromechanical transducer (for example linear motor, loudspeaker or the like) come up against limits without achieving complete uniformity. Moreover, such constructional measures frequently make the design more expensive and involve additional tolerance problems or a worsening of the data of such electric motors or transducers.
A different possible way of improving the synchronism is the electrical compensation of the torque pulsations. In the simplest case, an automatic control device ensures improved synchronism, running up or positioning. Further, the demands made on the controller with respect to adaptive control parameters, rapidity and stability cannot always be satisfactorily fulfilled with this method. Therefore, it is suitable to relieve the controller of the oscillatory moments and to generate the current harmonics required for constant torque in accordance with a characteristic line which has been previously determined from the motor data.
A method frequently used with brushless DC motors is to vary the ratio of the switch-on and switch-off range of the square-wave actuation. By means of a corresponding selection of the switch-on range of the different phases, an improved synchronism characteristic is achieved.
A universal and even better matched actuation is obtained by superimposing defined current harmonics. The required summing current curves can deviate considerably from a sinusoidal or square-wave shape. In this way, without external intervention in the motor, the synchronism quality can be considerably improved in a purely electronic manner. The motor developer is now presented with the possibility of optimizing the drive according to other viewpoints (for example, a more favorable production method). However, the greater outlay, in terms of control and power electronics, required for this should not be overlooked. The most recent progress in microelectronics and power transistors makes it considerably easier today to realize such high-quality servodrive systems.
Previous work on the aforementioned subject area is restricted merely to the electronic compensation of the electromagnetic torque fluctuations. This is usually only sufficient for an electric drive if the generated useful torque is very much greater than the other angle-dependent interfering pulses. Generally, this requirement is not fulfilled. Instead, a drive is required here which supplies in an angle-independent manner a constant moment over the entire torque range, i.e. a simultaneous reduction of permanent-magnet torque fluctuations, electromagnetic torque fluctuations, reluctance torque fluctuations and mechanical torque fluctuations or a selection of the latter if one or more components are negligible.
In addition, it is already known from EP-A-180 083 to generate a defined angle-dependent torque by means of corresponding actuation with currents of a particular curve shape. However, with this known measure only reluctance torque fluctuations are reduced, and to be precise also not very extensively since the actuation curves used are symmetrically trapezoidal or sinusoidal with a flattened maximum range.